Hydrogen-Mediated Electron Doping of Gold Clusters As Revealed by

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Hydrogen-Mediated Electron Doping of Gold Clusters as Revealed by In Situ X-ray and UV-Vis Absorption Spectroscopy Ryo Ishida, Shun Hayashi, Seiji Yamazoe, Kazuo Kato, and Tatsuya Tsukuda J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b00722 • Publication Date (Web): 01 May 2017 Downloaded from http://pubs.acs.org on May 11, 2017

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The Journal of Physical Chemistry Letters

Hydrogen-Mediated Electron Doping of Gold Clusters as Revealed by In Situ X-Ray and UV-Vis Absorption Spectroscopy Ryo Ishida,1 Shun Hayashi,1 Seiji Yamazoe,1,2 Kazuo Kato,3 and Tatsuya Tsukuda*1,2 1

Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Katsura, Kyoto 615-8520, Japan 3 Japan Synchrotron Radiation Research Institute, SPring-8, 1-1-1 Koto, Sayo, Hyogo 679-5198, Japan 2

Supporting Information ABSTRACT: We previously reported that small (~1.2 nm) gold clusters stabilized by poly(N-vinyl-2-pyrrolidone) (Au:PVP) exhibited a localized surface plasmon resonance (LSPR) band at ~520 nm in the presence of NaBH4. To reveal the mechanism of this phenomenon, the electronic structure of Au:PVP during the reaction with NaBH4 in air was examined by means of in situ X-ray absorption spectroscopy at Au L3-edge and UV-Vis spectroscopy. These measurements indicated that the appearance of the LSPR band is not associated with the growth in size, but is ascribed to electron doping to the Au sp band by the adsorbed H atoms. Gold clusters whose surfaces are partially stabilized by polymers or proteins have provided an ideal platform for studying the intrinsic stabilities1 and surface-mediated properties such as catalysis2–9 and sensing.10–12 Their unique properties are mainly governed by their electronic structures, which are significantly different from those of the corresponding bulk and nanoparticles (NPs). For example, small Au clusters show significantly different optical properties compared to Au NPs owing to the quantized electronic structures.13–15 Au clusters smaller than 2 nm show molecular-like optical absorption due to single-electron transitions between discrete electronic levels. In contrast, Au NPs larger than 2 nm exhibit localized surface plasmon resonance (LSPR) absorption, in which collective oscillation of the valence electrons is induced by incident light with a wavelength of ~520 nm. Recently, we observed that Au clusters stabilized by poly(N-vinyl-2-pyrrolidone) (Au:PVP) with an average diameter of ~1.2 nm exhibited an LSPR band only when BH4– coexisted in the dispersing media (Figure 1A).16 The LSPR band disappeared by the reaction with oxygen molecules. Ex situ TEM measurements revealed that the size of the Au clusters was retained before and after the reaction with NaBH4. This phenomenon was ascribed to the modulation of the electronic structure of Au clusters by the adsorption of hydrogen atoms originating from NaBH4. However, no direct spectroscopic evidence for the H-mediated modulation was demonstrated in the previous work. X-ray absorption fine structure (XAFS) measurement is a powerful tool for probing the geometric and electronic structural change of metal clusters during a reaction in solution. For example, it was reported that the intensity of the white line of Pt L2,3-edge X-ray absorption near-edge structure (XANES) spectroscopy was enhanced by the H adsorption on Pt NPs.17,18 This observation was attributed to the increased

probability of electronic transition from the Pt 2p orbital to unoccupied Pt 5d–H 1s antibonding bands18 based on the model by Hammer and Nørskov: the interaction between the H 1s orbital and Pt d band generated the bonding and antibonding states below the d band and above the Fermi level, respectively.19 Enhancement of the white line at the Au L3 edge for Au NPs supported on Al2O3 was also observed under H2 atmosphere while the extent of enhancement was smaller than that of Pt at the same H2 pressure due to the lower coverage with hydrogen.20,21 In this work, we performed in situ measurements of Au L3-edge XAFS and UV-Vis spectra of Au:PVP during the reaction with NaBH4 to correlate the behavior of the LSPR band with the electronic structure. It was found that the white line at the Au L3 edge intensified simultaneously with the appearance of the LSPR band whereas the coordination numbers of the Au–Au bonds did not change appreciably. We proposed that the appearance of the LSPR band observed in the reaction of Au:PVP and BH4– is ascribed to electron doping to the Au sp band from adsorbed H atoms. Au:PVP was prepared by the procedure reported previously.22 However, it was found that the concentration of Au:PVP employed for the original observation16 ([Au] = 1 mM) was too low to obtain XAFS spectra with a sufficient signal-to-noise ratio. As shown later (Figure 3A), Au:PVP with [Au] = 10 mM was used to obtain XAFS data that allowed us to quantitatively evaluate the small change in the white line intensity and to estimate the coordination numbers of the Au-Au bonds. Figure 1 shows how the UV-Vis spectra of Au:PVP with [Au] = 1 and 10 mM change with time after the addition of an aqueous solution of NaBH4 and K2CO3. In both cases, the LSPR band at ~520 nm emerged rapidly, reached a maximum, and decayed gradually with time. Insets of Figure 1 show the temporal profiles of absorbance at 520 nm measured at [Au] = 1 and 10 mM. Two features are found.

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Firstly, both the rise and decay rates for [Au] = 10 mM are smaller than those for [Au] = 1 mM. The reason for this phenomenon is that the concentration of dissolved O2 available for recovery to the initial state is lower at a higher concentration of Au:PVP. This explanation was supported by the significant acceleration of the decay process of the LSPR under pure O2 atmosphere (Figure S1). Secondly, the absorbance at ~520 nm did not return to the initial value at [Au] = 10 mM, in contrast to the case of [Au] = 1 mM. This suggests that the reaction of highly concentrated Au:PVP with NaBH4 results in a growth in size. However, the extent of this growth is negligibly small as demonstrated later (Figure 2B). Based on these considerations, in situ XAS measurements were performed using Au:PVP with [Au] =10 mM.

Figure 1. Time course of UV-Vis spectra of Au:PVP dispersion with [Au] = (A) 1 and (B) 10 mM after addition of NaBH4 and K2CO3 at t = 0 s. Insets show the temporal profiles of normalized absorbance at 520 nm.

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Au L3-edge XANES spectra of Au:PVP recorded in situ during the reaction are shown in Figure 2A. Although the extent of change is small, it is obvious from the difference spectra (Figure S2) that the intensity of the white line at 11918 eV rapidly increased soon after the addition of NaBH4, and then gradually decreased to the initial state. Note that the intensity at 11908 eV decreased irreversibly (Figure S2) although the reason for this observation is not understood at this moment. Figure 2B shows Fourier-transformed (FT) EXAFS spectra at the Au L3 edge before (t = –10 s), during (t = 225 s) and after the reaction (t = 2160 s) with NaBH4. The structure parameters obtained by analysis are listed in Table S1. The coordination numbers and lengths of the Au–Au bonds remained nearly unchanged by the reaction. These results indicate that the size growth suggested by Figure 1B was so small that it cannot be detected by EXAFS. Time courses of the intensities of the white line at 11918 eV and the LSPR band at 520 nm are compared in Figure 3. Both intensities started to increase at ~25 s, took the maximum values, and gradually diminished simultaneously from ~230 s. This result clearly shows that the enhancement of LSPR intensity was closely associated with that of the white line.

For the in situ measurements, a hydrosol of Au:PVP ([Au] = 10 mM) in a vessel was flowed in a closed circulatory system as shown in Scheme 1. Reaction was initiated (t = 0) by the rapid injection of an aqueous solution of NaBH4 into the dispersion of Au:PVP with a concentration ratio of [BH4–]:[Au] = 0.25:1. Decomposition of NaBH4 was suppressed by the coaddition of an aqueous solution of K2CO3. Measurement was initiated 60 s before the mixing (t = -60 s). It took ~15 and ~16 s for the mixed solution to reach the UV-Vis cell and X-ray tube, respectively (Scheme 2).

Scheme 1. Schematic Illustration of Experimental Setup Figure 2. (A) Time course of Au L3-edge XANES spectra of Au:PVP after addition of NaBH4. (B) FT-EXAFS spectra of Au:PVP at t = (a) –10, (b) 225 and (c) 2160 s.

Scheme 2. Timetable of Experiment

Since the white line at the Au L3 edge is assigned to the electronic transition from the occupied Au 2p to unoccupied Au 5d orbital, its intensity reflects the vacancy of the d orbitals.23–26 Thus, the increase and decrease of the white line intensity observed reversible changes in Au L3-edge XANES spectra indicate that the vacancy of d orbitals is decreased by the reaction with NaBH4 and increased by the reaction with dissolved O2, respectively. The increase of the white line intensity by the addition of NaBH4 excludes a possibility of simple reduction (electron donation) of Au:PVP. Such modulation of the electronic structure of Au:PVP can be explained by incorporating the interpretation of the Pt L3-edge XANES spectra of H-covered Pt NPs.18,27–30 As illustrated in Figure 4, the intensity of the white line is enhanced due to the formation of unoccupied Au 5d–H 1s antibonding bands. The


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formation of such antibonding bands conversely means the presence of bonding interaction between the H 1s orbital and Au 5d band. Consequently, the changes in XANES spectra during the reaction of Au:PVP with NaBH4 suggest that H atoms (or hydrides) originating from NaBH4 were chemically adsorbed on the surface of Au clusters via hybridization between the H 1s orbital and Au 5d band. We observed that Au:PVP (~1.2 nm) exhibited the LSPR band also by the reaction of with H2 (Figure S3). Recently, Bakr et al. reported generation of hydride-protected Ag clusters in the reaction of phosphine-protected Ag clusters with NaBH4.31 These are in line with the chemisorption of H atoms on Au:PVP under the reaction with NaBH4.

proposed in the reaction of Au nanorods with NaBH4.35 The electron density of Au clusters can be enhanced more efficiently by the adsorption of H atoms than by the addition of Au atoms because of much smaller volume of H atoms than Au atoms. Overall it can be viewed that Au provides d electrons for making the Au–H bond whereas the H atom dopes s electrons into the Au clusters. A similar charge compensation model has been employed to explain the charge transfer within Au-based alloy systems.36–41 In summary, we monitored the electronic structure and optical properties of PVP-stabilized Au clusters (~1.2 nm) during the reaction with NaBH4 by in situ measurements of XAFS and UV-Vis spectroscopy. Direct observation of the electronic structure of Au clusters revealed that H atoms originating from NaBH4 were adsorbed on the Au clusters. Therefore, the appearance of the LSPR band is ascribed to electron doping into Au clusters from H atoms while retaining the volume of the confined space.

EXPERIMENTAL SECTION

Figure 3. Time course of (A) normalized absorption at 11918 eV in Au L3-edge XANES spectra after the addition of NaBH4 and (B) optical absorption at 520 nm in UV-Vis spectra during the reaction of Au:PVP with NaBH4.

The synchronous changes in intensity of the LSPR band and the white line indicate that adsorbed H atoms induced the appearance of the LSPR band. In general, the LSPR involves the collective oscillation of electrons confined in metal NPs by the electromagnetic field of light.32 Theoretical and experimental studies showed that the intensity of the LSPR band depends mainly on the size of NPs, or in other words, on the number of electrons in valence bands.2,33 According to the photoelectron spectra of Aun– and Aun–1H–, an H atom electronically mimics a single Au atom and dopes one electron upon adsorption.34 This suggests that the LSPR band of Au:PVP is induced by the increase in electron density of the Au cluster due to electron doping from adsorbed H atoms into the Au cluster (Figure 4). A similar electron charging was

Figure 4. Schematic illustration of electronic structures of Au cluster before and after hydrogen adsorption.

Preparation of Au:PVP All chemicals were commercially available and used without further purification. Au:PVP were prepared as follows using a microfluidic mixer. Aqueous solutions of PVP with an average molecular weight of about 40 kDa (666.6 mg in 20 mL) and HAuCl4 (30 mM, 10 mL) were mixed in an ice bath of 273 K for 30 min. Another aqueous solution of PVP (666.6 mg in 25 mL) was cooled in an ice bath for 30 min and then an aqueous solution of NaBH4 (56.7 mg, 5 mL) was added to it after mixing for 5 min. These two solutions were injected from automatically actuated syringes at a rate of 200 mL/h into a micromixer (SIMM-V2, Institut für Mikrotechnik Mainz GmbH) placed in an ice bath. The eluted solution was collected and stirred at 273 K for 1 h. The hydrosol of Au:PVP thus prepared was deionized three times with water by using centrifugal ultrafiltration concentrators (Vivascience, Vivaspin20) having a membrane with a cutoff molecular weight of 10 kDa. UV-Vis spectroscopy in a circular flow system UV-Vis absorption spectra of Au:PVP dispersion during the reaction with NaBH4 or H2 in a circular flow system were recorded by a spectrophotometer (Agilent Technologies, Agilent 8453). The reaction with NaBH4 was conducted as follows. First, the Au:PVP (10 mM, 15 mL) dispersion in a flask cooled at 273 K was flowed by a pump into a quartz cell. Then, an aqueous solution of NaBH4 (1.42 mg) and K2CO3 (15 mg) dissolved in chilled water was added (t = 0). The reaction with H2 was conducted as follows. The Au:PVP (10 mM, 15 mL) dispersion in a sealed flask filled with N2 and kept at room temperature was flowed into the quartz cell. Then, H2 gas (0.1 MPa) was introduced into the flask (t = 0) and the flask was exposed to air after 3600 sec. In situ measurements of XAFS and UV-Vis spectra All measurements were performed using a circular flow system as shown in Scheme 1. The Au:PVP solution in the flask cooled at 273 K was flowed by a peristaltic pump into a quartz cell for UV-Vis absorption measurement and a subsequent fluoroplastic tube for X-ray absorption measurement. The aqueous solution of NaBH4 and K2CO3 dissolved in cooled water was automatically injected at t = 0 after starting in situ measurements. The time course of absorbance at 520 nm for Au:PVP was recorded with a spectrophotometer (Ocean Optics, USB2000+). DXAFS (dispersive XAFS) measurement of the Au L3 edge was



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conducted at the BL28B2 beamline in the SPring-8 facility of the Japan Synchrotron Radiation Research Institute. The DXAFS system was composed of a polychromator and a position-sensitive detector. The XAFS spectra were measured at room temperature in transmission mode. The X-ray energy was calibrated from the spectrum of Au foil. The exposure time of the position-sensitive detector was 89 ms and 100 shots were accumulated (8.9 s per spectrum). The Au L3-edge XANES and EXAFS measurements were performed independently. XANES spectra were obtained every 8.9 s with the use of the same I0 measured before the sequential DXAFS measurement. EXAF spectra were obtained every ~1 min with the use of each I0 measured right after each EXAFS spectrum measurement. Data analysis was carried out using Athena (Demeter Co.) and REX2000 (Rigaku Co.) software.

ASSOCIATED CONTENT Supporting Information. Details on syntheses, characterization, measurements, and analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E–mail: [email protected]

ACKNOWLEDGMENT This research was financially supported by the Elements Strategy Initiative for Catalysts & Batteries (ESICB) and by a Grant-in-Aid for Scientific Research (No. 26248003) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan. XAFS measurements at the SPring-8 facility were performed with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2016A1644, 2016B0910 and 2016B1493).

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Hydrogen-Mediated Electron Doping of Gold Clusters As Revealed by

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